Pneumatic manipulators and balancers have been used for many years in industry for repetitive lifting of loads. A manipulator includes an arm that lifts a load using a lift cylinder to keep the arm and the load up. A balancer is a hoist that uses a piston moving though a cylinder to lift the load. An end effector hangs from the manipulator or balancer, and incorporates a gripper, vacuum cups, or magnet to hold the load. A human operator operates controls on the end effector to grip or release the load. A problem is that there exists the possibility of the operator accidentally pressing the release button(s) while the load is being carried, resulting in the load falling and causing injury.
A load-sensing device is routinely installed in manipulators and balancers to sense the pressure in the lift cylinder and disable the release function. That is, while the load is being carried, the load-sensing device senses that the pressure in the lift cylinder (or balancer) is higher than it is in the no-load situation, and it prevents the release circuit from operating. Typically, the load-sensing device is an adjustable pneumatic pressure-sensing valve. When pressure rises above an adjustable threshold, or setpoint, an air signal changes state, which can then be used to disable release. The threshold is set manually with a dial or screw to a level somewhat higher than the no-load pressure. When the load is brought down to rest on a support, the lift pressure drops below the threshold to the no-load range, and the gripper can be released.
There are several drawbacks to existing available pneumatic pressure-sensing valves that make them unreliable:
Over time, the adjustable threshold can drift. Over a matter of days or months, the threshold can rise or fall slightly, due to vibration, contaminants in the air supply, repetitive flexing of internal components, and other factors. The result of a slight drop in the threshold is the permanent disabling of release, and a stop in production. If the threshold rises slightly, release can happen too early, making accidental dropping of parts a possibility.
Sometimes the threshold is not repeatable. For example, on one trial, the threshold might be 52 psi, but on the next it might be 54 psi.
Some load-sensing valves have a large hysteresis. For example, when the pressure is rising the output turns on at 61 psi, but when the pressure is falling, the output turns off again at 56 psi.
Some pressure-sensing valves switch states if the pressure very suddenly changes. For example, if the load is bounced up and down while being carried, the pressure-sensor may briefly allow a release to occur.
Pressure-sensing valves have to be adjusted by a person who understands how to make an accurate setup. If the no-load pressure changes, or if the setpoint has drifted, it will be up to a mechanic in the end user's factory to make this adjustment, which can lead to errors, and a loss in production.
Pneumatic pressure-sensing valves cannot reliably detect very light loads. If an end effector weighs 120 lb, the heaviest load weighs 100 lb, and the lightest weighs 20 lbs, typically the load-sensing valve will not be able to detect the presence or absence of the light load. The no-load pressure (that is, the pressure required to suspend an empty end effector) may be 50 psi, and the pressure required to lift the lightest load may be 54 psi. This 4 psi difference cannot be reliably detected by some pneumatic pressure-sensing valves.